Low-speed bus time stamp methods and circuitry

ABSTRACT

Methods and circuitry for low-speed bus time stamping and triggering are presented in this disclosure. A master device and slave devices can be interfaced via a communication link that comprises a data line and a clock line. The master device generates and controls a clock signal on the clock line, and sends a synchronization command over the data line to the slave devices. In response to the synchronization command, the master device receives timestamp information of an event detected at each slave device. The master device tracks transitions and frequencies of the clock signal, and determines a time of the event based on the timestamp information, the tracked transitions and the frequencies. The master device can further send to each slave device delay setting information for generating a trigger signal at that slave device based on transitions of the clock signal, the synchronization command and the delay setting information.

BACKGROUND

The present disclosure generally relates to a low-speed bus message protocol, and more particularly relates to methods and circuitry for low-speed bus time stamping and triggering.

Inter-Integrated Circuit (I2C) interface is typically used for attaching lower-speed peripheral Integrated Circuits (ICs) to higher-speed processors and microcontrollers. Lower-speed peripheral ICs are commonly referred to as slave devices, whereas a higher-speed processor or microcontroller is commonly referred to as a master device. Often, a slave device can be coupled to a peripheral device such as a sensor, a gyroscope, a compass, a microphone, and the like. The slave device can be configured to monitor and/or control operations of the peripheral device coupled to the slave device.

In I2C message protocol, a simultaneous operation by two or more slave devices can utilize a common trigger signal (e.g., generated by a master device), which is independent of an I2C low-speed serial bus. Similarly, in order to determine when an event occurred (e.g., measurement performed by a peripheral device coupled to a slave device), each slave device uses a dedicated line feeding back to the master device for signaling to the master device a time when the event occurs. For each slave device, the master device can capture a state of a real time clock (i.e., time of event, or timestamp of event) when the master device receives an event marker signal from the slave device. The disadvantage of this approach is a number of additional communication lines (i.e., board traces) between the master device and the slave devices, and additional signal pins that are required.

SUMMARY

Certain embodiments of the present disclosure provide a system. The system generally includes a master device coupled to a communication link, the master device to transmit, via the communication link, a clock signal and a synchronization command, and one or more slave devices coupled to the communication link, each slave device to track a number of selected transitions of the clock signal between the synchronization command and an event detected at that slave device, and generate information about an elapsed time between the synchronization command and the event detected at that slave device, the information based on the number of selected transitions of the clock signal tracked at that slave device, and wherein the master device to obtain the information about the elapsed time and to derive a time the event was detected at that slave device.

Certain embodiments of the present disclosure provide an apparatus. The apparatus generally includes an interface circuit for coupling to a communication link, the interface circuit to transmit, via the communication link, a clock signal and a synchronization command, and receive, via the communication link, timestamp information indicative of a number of selected transitions of the clock signal that elapse between the synchronization command and a time instant when an event is detected at a slave device, a time tracking circuit to track counts of selected transitions of the clock signal between the synchronization command and frequency changes of the clock signal occurring after the synchronization command, and a time calculation circuit to determine a time of the event detected at the slave device based on the timestamp information and the counts of the selected transitions of the clock signal.

Certain embodiments of the present disclosure provide an apparatus. The apparatus generally includes an interface circuit for coupling to a communication link that carries a clock signal, and a control circuit to track a number of selected transitions of the clock signal on the communication link between a synchronization command received via the communication link and a detection of an event, the interface circuit to transmit, via the communication link, information about an elapsed time between the synchronization command and the detection of the event, the information based on the number of selected transitions of the clock signal.

Certain embodiments of the present disclosure provide a method. The method generally includes generating a clock signal and a synchronization command, transmitting, via a communication link, the clock signal and the synchronization command, receiving, via the communication link, timestamp information indicative of a number of selected transitions of the clock signal that elapse between the synchronization command and a time instant when an event is detected at a slave device, tracking counts of selected transitions of the clock signal between the synchronization command and frequency changes of the clock signal occurring after the synchronization command, and determining a time of the event detected at the slave device based on the timestamp information and the counts of the selected transitions of the clock signal.

Certain embodiments of the present disclosure provide an apparatus. The apparatus generally includes an interface for coupling to a communication link that carries a clock signal, the interface to receive via the communication link a synchronization command and first delay setting information, and a control circuit to track a number of selected transitions of the clock signal after the synchronization command and to generate a trigger signal responsive to the number of selected transitions reaching a delay setting indicated by the first delay setting information.

Certain embodiments of the present disclosure provide a method. The method generally includes receiving, via a communication link that carries a clock signal, a synchronization command and first delay setting information, tracking a number of selected transitions of the clock signal after the synchronization command, and generating a trigger signal responsive to the number of selected transitions reaching a delay setting indicated by the first delay setting information.

Certain embodiments of the present disclosure provide an apparatus. The apparatus generally includes an interface circuit for coupling to a communication link, the interface circuit to transmit, via the communication link, a clock signal and a synchronization command, and transmit, via the communication link, delay setting information indicating a number of selected transitions of the clock signal that are to occur between the synchronization command and generation of a trigger signal at one or more slave devices coupled to the communication link.

Certain embodiments of the present disclosure provide a method. The method generally includes transmitting, from a master device via a communication link, a clock signal and a synchronization command, and transmitting, via the communication link, delay setting information indicating a number of selected transitions of the clock signal that are to occur between the synchronization command and generation of a trigger signal at one or more slave devices coupled to the communication link.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram that illustrates I3C master device interfaced with multiple slave devices via I3C based communication link, in accordance with embodiments of the present disclosure.

FIG. 2 is a schematic diagram of a system comprising a master device interfaced with a slave device via I3C based communication link for enabling time stamping and delayed triggering, in accordance with embodiments of the present disclosure.

FIG. 3 illustrates an example time stamp synchronization command and waveforms of signals driving I3C serial buses in relation to the time stamp synchronization command, in accordance with embodiments of the present disclosure.

FIG. 4 is an example schematic of circuitry for implementing time synchronization at a slave device, in accordance with embodiments of the present disclosure.

FIG. 5 is an example schematic of an oscillator circuit that may be implemented at a slave device for improving resolution of time synchronization, in accordance with embodiments of the present disclosure.

FIG. 6 is an example schematic of circuitry for implementation of time-stamping that may be implemented at a slave device, in accordance with embodiments of the present disclosure.

FIG. 7 is an example diagram of capturing and reading time of events by a master device from multiple slave devices, in accordance with embodiments of the present disclosure.

FIG. 8 is an example diagram of capturing and reading time of events by a master device and/or a monitor device from multiple slave devices, in accordance with embodiments of the present disclosure.

FIG. 9 is an example schematic of circuitry for implementation of time-stamping at a slave device without an oscillator circuit (e.g., the oscillator circuit from FIG. 5), in accordance with embodiments of the present disclosure.

FIG. 10 is an example schematic of circuitry for implementation of delayed triggering at a slave device, in accordance with embodiments of the present disclosure.

FIG. 11 is an example diagram of controlling time of events by a master device at multiple slave devices, in accordance with embodiments of the present disclosure.

FIG. 12 is an example schematic of circuitry that may be implemented at a master device for supporting time stamping, in accordance with embodiments of the present disclosure.

FIG. 13 is a diagram illustrating a method performed at a master device for time stamping changes in a reference clock signal, in accordance with embodiments of the present disclosure.

FIG. 14 is a flow chart illustrating a method for time stamping that may be performed at a master device, in accordance with embodiments of the present disclosure.

FIG. 15 is a flow chart illustrating a method for delayed triggering that may be performed at a slave device, in accordance with embodiments of the present disclosure.

FIG. 16 is a flow chart illustrating a method for delayed triggering that may be performed at a master device, in accordance with embodiments of the present disclosure.

The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

Embodiments of the present disclosure relate to synchronizing multiple slave devices operating in conjunction with a master device in accordance with a messaging protocol, such as the I3C message protocol, which is an enhanced version of the Inter-Integrated Circuit (I2C) message protocol. Synchronization of multiple slave devices presented herein can provide accurate time stamping of events detected at the slave devices, as well as efficient initiation of delayed triggered events at the multiple slave devices.

Certain embodiments of the present disclosure support initiating simultaneous readings/operations of peripheral devices coupled to the slave devices. For example, methods and circuitry presented herein can synchronize measurements between a gyroscope and a magnetic compass (that are coupled to a pair of slave devices), while both the gyroscope and the magnetic compass are located on a rotating object. The methods and circuitry presented in this disclosure can also initiate delay triggered events on multiple slave devices, which can be useful for tomography.

In accordance with embodiments of the present disclosure, multiple slave devices can initiate simultaneous operations (e.g., measurements) via I3C time synchronization triggering, as discussed in more detail below. In this way, the need for side channels to synchronize events can be eliminated. There is no concern for time units or local clock signals since all slave devices can be triggered simultaneously. More generally, embodiments of the present disclosure support usage of a time synchronization command that starts a timer at each slave device that triggers an event at an end of a pre-determined time period. A time delay for a triggering event at each slave device can be set by a directed command that may precede the time synchronization command.

In the illustrative embodiment of the present disclosure, cell-phone based tomography can be considered. Each slave device may drive one transducer of an array of transducers (e.g., located at a back of a cellular phone), wherein the transducer generates an acoustic pulse (e.g., based on a trigger signal from the slave device) at the end of the aforementioned individual time delay interval (e.g., to control phase for beam-forming). Shortly thereafter, each transducer may receive a reflected waveform, wherein each feature of the reflected waveform (e.g., that is within a preset time aperture and within a present magnitude/derivative/second derivative limits, as defined by an earlier command) can be time-stamped, which is recorded in a register at the slave device. The master device may then poll each slave device and read back the stored time-stamped data. For example, after a certain number of triggering/time-stamping operations, there is sufficient operation to make an image of an interior of abdomen (or some other internal organ).

In accordance with embodiments of the present disclosure, independent clock signals and counter circuits in different slave devices can be synchronized that are used to time-stamp their readings. In this way, events from different sensors can be accurately correlated in time. For example, a plurality of measurements produced by an array of I3C microphones can be correlated to determine a direction from which a sound (e.g., “clap”) originates, wherein each microphone in the array can have its own clock signal.

Embodiments of the present disclosure support utilizing a new common command code (CCC) serial bus command. i.e., “Time Sync” command for time synchronization. In some embodiments, a master device may issue Time Sync CCC to synchronize all slave devices to a particular selected transition (e.g., falling edge) of a clock signal driving a Serial Clock Line (SCL) bus. Each slave device may be configured to count all selected transitions of SCL signal after Time Sync CCC is detected, and may use selected transitions of SCL clock signal as time markers for time-stamping events. The master device may count all selected transitions of SCL clock signal after detecting Time Sync CCC while also monitoring a period of transitions of SCL dock signal against a (stable) time base. The master device may also monitor bus traffic for time-stamp data, collect the time-stamp data and perform calculations to determine timing of events (e.g., sensor measurements) detected at the slave devices against the time base. In other embodiments a monitor device separate from the master device may perform the counting of SCL, transitions and collection of time stamped data.

Embodiments of the present disclosure facilitate accurate time-stamping and triggering. In one or more embodiments, for time-stamping, a slave device may monitor a sensor and record a time (count) that a sensed event occurs. In one or more other embodiments, for triggering, a master device may issue a command for all slave devices in a group to initiate certain operations at a precise time (count). It should be noted that this may be initiation of a time-delay after which an action occurs, wherein the time-delay may be preset to different delay values on a per slave device basis.

FIG. 1 is a schematic diagram 100 that illustrates a master device 102 interfaced with multiple slave devices 104, in accordance with embodiments of the present disclosure. In one or more embodiments, each slave device 104 may be a lower-speed peripheral integrated circuit (IC), whereas the master device 102 may be a higher-speed processor or microcontroller. In an embodiment, the master device 102 may be coupled to a real time clock source 106 that generates a clock signal 108 for the master device 102. In another embodiment, the master device 102 may comprise an internal clock signal source for generating a clock signal.

As illustrated in FIG. 1, the master device 102 may be interfaced with the slave devices 104 via communication link 110. In some embodiments, the communication link 110 is a two wire communication link that comprises a serial data line (SDA) bus 112 and SCL bus 114. SDA bus 112 is a single wire bus that may be employed to carry commands and/or data between the master device 102 and the slave devices 104 using single ended signals in accordance with a communication protocol such as I3C. SCL bus 114 is a single wire bus that may be utilized to carry a single-ended clock signal (e.g., the clock signal 108) that may be generated and/or controlled by the master device 102. Clock signal 108 is used as a timing reference for transmitting and receiving commands and/or data on the SDA bus 112. Each slave device 104 may be coupled to a peripheral device (e.g., transducer, microphone, sensor, and the like) controlled by that slave device 104.

For some embodiments, as discussed in more detail below, the master device 102 may issue a time synchronization command via SDA bus 112 to synchronize local counts of selected transitions of clock signals (e.g., falling edges of clock signals) in different slave devices 104 in order to accurately time-stamp readings (events) from devices (e.g., sensors) coupled to the slave devices 104. The time-stamped events locally stored at each slave device 104 may be provided (e.g., via SDA bus 112) to the master device 102 for calculation of a real time occurrence of each event, wherein a global real time can be accurately tracked by the master device 102 based on transitions of the clock signal 108 (e.g., signal carried by SCL bus 114). In this way, events (e.g., measurements) from different sensors coupled to different slave devices 104 can be accurately correlated in time at the master device 102.

For some other embodiments, as discussed in more detail below, multiple slave devices 104 can initiate synchronized operations (e.g., measurements) via time synchronization triggering controlled by the master device 102 (e.g., by sending an appropriate command via SDA bus 112). Thus, the need for side communication channels between the master device 102 and the slave devices 104 for synchronization of operations (events) can be eliminated.

FIG. 2 is a schematic diagram of a system 200 comprising a master device 202 interfaced with a slave device 204 via a communication link 205, which may enable time stamping and delayed triggering, in accordance with embodiments of the present disclosure. For some embodiments, the master device 202 may correspond to the master device 102 shown in FIG. 1, and the slave device 204 may correspond to any of the slave devices 104 shown in FIG. 1. Although one slave device 204 is illustrated in FIG. 2, embodiments of the present disclosure support interfacing multiple slave devices 204 to the master device 202. As illustrated in FIG. 2, the communication link 205 may comprise SDA bus 206 and SCL bus 208. As further illustrated in FIG. 2, the master device 202 and the slave device 204 may both drive SDA bus 206, whereas only the master device 202 may provide and control a clock signal that may be carried by SCL bus 208 (hereinafter referred as SCL clock signal 208). The slave device 204 may communicate with the master device 202 via SDA bus 206, and the slave device 204 may utilize SCL clock signal 208 for time-stamping of an event detected by the slave device 204 and/or for synchronized delayed triggering, as discussed in more detail below.

In some embodiments, the master device 204 may broadcast via SDA bus 206 a Single Data Rate (SDR) command 210 to the slave device 204. In one or more embodiments, the SDR command 210 may comprise a Time Sync CCC. Upon decoding of the SDR command 210 (e.g., by decode logic 212), the slave device 204 may operate in accordance with the decoded SDR command 210. In an embodiment, Time Sync CCC may be detected at a Time Tracking/Trigger Control circuit 214. Based on the detected Time Sync CCC, a Time Sync Marker (not shown) may be generated by the Time Tracking/Trigger Control circuit 214 for start of time synchronization and time tracking until an event occurs and is detected, as discussed in more detail below. As illustrated in FIG. 2, the slave device 204 may be coupled to a sensor 216 that generates a sensor output signal 218 indicative of a measurement of an environmental property. An event detector circuit 220 detects occurrence of an event from the sensor output signal 218, and generates an event detection signal 222 that switches from a low logic level to a high logic level when the event is detected.

In some embodiments, the Time Tracking/Trigger Control circuit 214 may be configured to time-stamp occurrence of the event (e.g., sensor measurement) 222 with reference to the start of time synchronization, which may be indicated by the Time Sync Marker (not shown). As discussed in more detail below, the Time Tracking/Trigger Control circuit 214 may perform time-stamping of the event 222 based at least in part on selected transitions of SCL clock signal 208 (i.e., reference clock signal) that may be generated and controlled by the master device 202. The Time Tracking/Trigger Control circuit 214 may store a time stamp 224 of the event 222. As illustrated in FIG. 2 and discussed in more detail below, a communication logic 226 may read a value of the time stamp 224 and provide the time stamp value 224 to SDA bus 206 (e.g., when SDA bus 206 is free from other traffic). As further illustrated in FIG. 2, the communication logic 226 and the decode logic 212 represent an interface 228 that couples the slave device 204 to SDA bus 206.

In some embodiments, prior to broadcasting the SDR command 210 with Time Sync CCC, the master device 202 may communicate (e.g., via SDA bus 206) other SDR command(s) to the slave device 204 with delay setting information that determines a time delay for generating a trigger signal by the slave device 204. As illustrated in FIG. 2, a trigger delay setting circuit 230 generates delay setting information 232, which indicates a trigger delay in the form of a number of selected transitions of SCL clock signal 208 that are to occur between the SDR command 210 with the Time Sync CCC and generation of the trigger signal at the slave device 204. In an embodiment, the trigger delay setting circuit 230 generates delay setting information 232 based on expected frequency changes of SCL clock signal 208 that are to occur after the SDR command 210 with the Time Sync CCC. Information about the expected frequency changes of SCL clock signal 208 are known at the master device 202. An encoder 234 of a master device communication interface 236 encodes the delay setting information 232 within the SDR command 210. The SDR command 210 with the encoded delay setting information 232 is then broadcast via SDA bus 206 to one or more slave devices 204 to initiate delayed trigger. As further illustrated in FIG. 2, once the decode logic 212 of the slave device 204 decodes the delay setting information provided by the master device 202 within the SDR command 210 (e.g., coarse and fine delay settings) followed by the detection of Time Sync CCC encoded in another SDR command 210, the Time Tracking/Trigger Control circuit 214 may be configured to generate a delayed trigger signal 238 with a time delay determined based on the provided delay setting information, as discussed in more detail below. In an embodiment, the delayed trigger signal 238 may initiate operation (e.g., measurement) of a peripheral device coupled to the slave device 204, e.g., operation of an output transducer 240 coupled to the slave device 204.

In some embodiments, a time tracking circuit 242 of the master device 202 illustrated in FIG. 2 may be configured to track real time starting from a Time Sync Marker generated upon Sync signal 244. The encoder 234 encodes Sync signal 244 to generate the SDR command 210 with Time Sync CCC, which may be then broadcast via SDA bus 206 to one or more slave devices 204 to initiate time synchronization. Sync CCC broadcast 246 (i.e., Time Sync CCC) may be also detected within the Time Tracking circuit 242, which may then generate the Time Sync Marker that indicates a start of tracking a system reference time at the master device 202 based on tracking a number of selected transitions of SLC clock signal 208.

In some embodiments, a counter circuit 248 within the Time Tracking circuit 242 may be configured to keep track of the number of selected transitions (e.g., falling edges) of SCL clock signal 208. For each frequency of SCL clock signal 208, a number of selected transitions of SCL signal 208 (e.g., denoted in FIG. 2 as SCL count C0) may be saved into a latch 250, which may be controlled by a change of frequency (COF) signal 252. As discussed in more detail below, SCL count C0 may represent a number of selected transitions of SCL clock signal 208 between the Time Sync Marker and a last selected transition (e.g., falling edge) of SCL clock signal 208 prior to a change of frequency of SCL clock signal 208. After every change of frequency of SCL clock signal 208, an updated SCL count C0 may be stored in the latch 250, which is controlled by COF signal 252. The updated SCL count C0 may indicate a number of selected transitions of SCL clock signal 208 between the Time Sync Marker and a last selected transition of SCL clock signal 208 prior to a change of frequency of SCL clock signal 208. Upon every change of frequency of SCL clock signal 208 and based on corresponding COF signal 252, a previous (old) value of SCL count C0 may be also saved in a register file (e.g., look-up table) 254. Thus, the register file 254 may include different values of SCL count C0 (e.g., values CNT_1, CNT_2, . . . , CNT_N) that correspond to N different frequencies of SCL clock signal 208. Each value CNT_i stored in the register file 254 may be also associated with a value Ti that encodes a period of each frequency of SCL clock signal 208. Therefore, values of CNT_i and Ti (i=1, . . . , N) stored in the register file 254 may provide information about a system reference time from the Time Sync Marker.

In some embodiments, the master device 202 may receive, via SDA bus 206, information about the time stamp 224 of the event 222 detected at the slave device 204. The master device 202 may use information stored in the register file 254 about the system reference time tracked from initiation of the Time Sync Marker to correlate it with the time stamp 224 (e.g., at real time calculation circuit 256) to determine an exact global (system) time 258 of occurrence of the event 222. The calculated time 258 represents a global time that is measured based on selected transitions of SCL clock signal 208 starting from initiation of the Time Sync Marker at the master device 202. In an embodiment, SCL clock signal 208 may be generated at the master device 202 by an adjustable clock generator 260, which may provide a frequency of SCL clock signal 208 based on indication 262 (e.g., indication Ti) about a desired period of SCL clock signal 208.

As discussed above, embodiments of the present disclosure support adding a new Time Stamp Sync CCC broadcast command into a message protocol. The master device 202 may issue Time Stamp Sync command via SDA bus 206 to synchronize one or more slave devices 204 coupled to SDA bus 206 to a particular selected transition (e.g., falling edge) of a clock signal driving SCL bus. FIG. 3 illustrates an example Time Stamp Sync command 300 and waveforms of signals driving SDA and SCL buses in relation to the Time Stamp Sync command 300, in accordance with embodiments of the present disclosure. Time Stamp Sync command 300 may be initiated by the master device 202 and broadcast to one more slave devices 204 via SDA bus 206. As illustrated in FIG. 3, a start portion 302 of Time Stamp Sync command 300 may be followed by a Broadcast portion 304 indicated with value 0x7E). Towards the end of the Broadcast portion 304, the master device may signal a write operation (‘W’) to the slave device(s), wherein at least one slave device may respond to the write operation (‘W’) on SDA bus with an Acknowledgement (ACK), to acknowledge reception of the Broadcast portion 304 of Time Stamp Sync command 300.

As illustrated in FIG. 3, SDR command CCC portion 306 of Time Stamp Sync command 300 may follow the Broadcast portion 304. Command code 0x28 corresponds to a Time Stamp Sync command. A portion 308 (e.g., ‘T’ bit) may be associated with a specific signal waveform 310 on SDA bus. During ‘T’ bit of Time Stamp Sync command 300, on a first selected transition (e.g., rising edge) of SCL clock signal, the slave device 204 may detect Time Sync CCC 312. The next selected transition (e.g., falling edge) of SCL clock signal may represent a Time Sync Marker 314, which is also detected at the slave device 204. As discussed in more detail below, the Time Sync Marker 314 may represent a time instant when synchronization of one or more slave devices 204 with a system reference time base produced by the master device 202 starts. As further illustrated in FIG. 3, Time Stamp Sync command 300 may end with a portion 316 that initiates reading of data from the slave devices 204 via SDA bus.

In some embodiments, as discussed in more detail below, the Time Sync Marker 314 provides a means for multiple slave devices to synchronize for timestamping events. The Time Sync Marker 314 also allows multiple slave devices to initiate simultaneous operations (e.g., measurements) via Time Sync Triggering. As a result, the need for side channels between a master device and slave devices to synchronize events can be eliminated. It should be noted that in the triggering case there is no concern for time units or local clocks since all slave devices are triggered simultaneously.

In some other embodiments, time-stamping of an event detected at a slave device may be supported based on the Time Sync Marker 314. As discussed in more detail below, a control circuit within the slave device may be initialized based on the Time Sync Marker 314, and may be configured to track a number of selected transitions of SCL clock signal. Once an event is detected, the number of tracked selected transitions of SCL clock signal may be saved in a slave device's local memory to be read back by a master device at a later time. The master device, which generates and controls the SCL clock signal, may also keep track of a number of selected transitions of the SCL clock signal, and may correlate its count with the saved time stamp count read back from the slave device in order to determine a global system time of occurrence of the event.

FIG. 4 is an example schematic of circuitry 400 for implementing time synchronization at a slave device, such as slave device 204, in accordance with embodiments of the present disclosure. In some embodiments, the circuitry 400 may be a part of the Time Tracking/Trigger Control circuit 214 illustrated in FIG. 2. A flip flop 420 outputs a sync pulse 402 onto reset line 404 when (Time) Sync CCC is detected, i.e., when the rising edge of pulse 406 is detected. Referring back to FIG. 2, Sync CCC Detected pulse 406 may be generated by the decode logic 212 of the slave device 204 upon detection of a time synchronization command 210. A selected transition of the sync pulse 402, which is falling edge 408 as shown in FIG. 4, may represent the Time Sync Marker. Referring back to FIG. 3, the Time Sync Marker 314 may align with a selected transition of SCL clock signal during the ‘T’ bit of Time Stamp Sync command 300 following the detection of Time Sync CCC. Thus, as illustrated in FIG. 4, the Time Sync Marker may align with a selected transition 410 of SCL clock signal 412 following the rising edge of the pulse 406 indicating detection of Sync CCC.

In some embodiments, the sync pulse 402 present at the reset line 404 may reset a counter 414 to all zeroes, as illustrated by waveforms 416 at the output of the counter 414. The counter 414, after being reset to all zeroes, increments on every selected transition (e.g., on every falling edge) of SCL clock signal 412. It can be noted that the approach presented herein and illustrated in FIG. 4, which is based on the sync pulse 402 and the Time Sync Marker aligned with a selected transition of SCL clock signal (which can be controlled by a master device) provides a uniform time reference across all slave devices comprising the circuitry 400 shown in FIG. 4.

In some embodiments, a burst oscillator may be employed at a slave device to improve resolution of time-stamping and delayed triggering. FIG. 5 is an example schematic of an oscillator circuit 500 that may be implemented at a slave device 204 for improving resolution of time synchronization, in accordance with embodiments of the present disclosure. In one or more embodiments, the oscillator circuit 500 may be a part of the Time Tracking/Trigger Control circuit 214 of the slave device 204 shown in FIG. 2.

As illustrated in FIG. 5, the oscillator circuit 500 may comprise a burst oscillator 502 and a counter 504. The burst oscillator 502 includes several serially connected inverters that generate a high speed clock signal 506 when enable signal 508 is at a high logic level. A frequency of the high speed clock signal 506 is higher than a frequency of the SCL clock signal. Upon initiation by a reset signal 510, the counter 504 starts counting selected transitions of the high speed clock signal 506. Output F(0) of the burst oscillator 502 and m bit outputs F(1:m) of the counter 504 form an output 512 of the oscillator circuit 500. In one or more embodiments, the burst oscillator 502 may be configured to operate for a limited amount of time sufficient to make a certain number of measurements (e.g., one or two measurements) following detection of an event. Thus, the burst oscillator 502 consumes a limited amount of power.

In one embodiment, certain type of sensors (e.g., accelerometers, gyros) coupled to slave devices inherently have a relatively stable time base, and may use this time base to provide a clock signal that may be utilized to improve resolution of time-stamping and delayed triggering. Other sensors may not have stable time base and need to employ a local oscillator for generating a local clock signal. In an embodiment, the local oscillator at a slave device may be based on Phase Locked Loop (PLL) device that uses SCL clock signal as a reference clock to generate a synchronized and stable local clock of a higher frequency than SCL clock signal. However, this approach has the drawback of consuming continuous power and large silicon area.

FIG. 6 is an example schematic of circuitry 600 for implementation of time-stamping at a slave device 204 in accordance with embodiments of the present disclosure. The circuitry 600 may be a part of the Time Tracking/Trigger Control circuit 214 of the slave device 204 shown in FIG. 2. As illustrated in FIG. 6, the circuitry 600 may comprise the circuitry 400 from FIG. 4 and the oscillator circuit 500 from FIG. 5. In some embodiments, the circuitry 600 may be configured to implement a time stamp at a slave device 204, and the oscillator circuit 500 is utilized to increase resolution of the time stamp when compared to using only counts of selected transitions of SCL clock signal for the time stamp.

As discussed above with reference to the circuitry 400 illustrated in FIG. 4, a sync pulse (e.g., the sync pulse 402 shown in FIG. 4) present at a reset line 602 may be generated when (Time) Sync CCC is detected, i.e., when a rising edge of pulse 406 shown in FIG. 4 is detected at an input 604. A falling edge of the sync pulse (e.g., the sync pulse 402 shown in FIG. 4) may represent the Time Sync Marker that aligns with a selected transition (e.g., falling edge 410 shown in FIG. 4) of SCL clock signal 606 during ‘T’ bit of Time Stamp Sync command (e.g., SDR Time Sync command 210 broadcast from the master device 202 shown in FIG. 2, Time Stamp Sync command 300 shown in FIG. 3) following the detection of Sync CCC at the input 604. The sync pulse present at the reset line 602 may reset a counter 608 to all zeroes. In an embodiment, the counter 608 may be the same counter 414 of the circuitry 400 shown in FIG. 4. The counter 608 may be configured to increment on every selected transition (e.g., falling edge) of SCL clock signal 606, and may provide a uniform time reference across all slave devices (e.g., slave devices 104 illustrated in FIG. 1, multiple slave devices 204 shown in FIG. 2), wherein SCL clock signal 606 may be generated and controlled by a master device (e.g., the master device 102 shown in FIG. 1, the master device 202 shown in FIG. 2).

In some embodiments, an event 610 may be time stamped based at least in part on a value 612 of the counter 608. Upon detecting occurrence of the event 610, the value 612 representing a number of selected transitions of SCL clock signal 606 between the Time Sync Marker and a last selected transition 614 of SCL clock signal 606 prior to detection of the event 610 may be stored in a latch 616 (e.g., the value C0 shown in FIG. 6 may be stored in the latch 616).

In some embodiments, as discussed, the oscillator circuit 500 may be used in conjunction with the counter 608 to provide finer resolution for time-stamping. The oscillator circuit 500 comprising a burst oscillator 502 from FIG. 5 may be configured to generate a periodic oscillator signal having a frequency higher than a frequency of SCL clock signal 606. As illustrated in FIG. 6, upon detection of the event 610, flip flop 618 generates an enable signal 620 that activates the burst oscillator 502 within the oscillator circuit 500. Upon the activation based on the enable signal 620, the burst oscillator 502 of the oscillator circuit 500 may generate a high speed clock signal (oscillator signal) 506, and the counter 504 of the oscillator circuit 500 may keep track of a number of selected transitions (e.g., falling edges) of the oscillator signal 506.

In one or more embodiments, a first selected transition 622 of SCL clock signal 606 immediately following detection of the event 610 causes the output of flip flop 624 to go high, thereby initiating storage of a value 626 at the output of the oscillator & counter circuit 500 in a latch 628. This value is shown as C1. The value of C1 represents a delay, in the form of a number of selected transitions of the oscillator signal 506, between detection of the event 610 and the first selected transition 622 of SCL clock signal 606 following the detection of the event 610.

A next selected transition 630 of SCL clock signal 606 following the first selected transition 622 causes the output of flip flop 632 to go high. As a result, this initiates storage of a new value 626 at the output of the oscillator & counter circuit 500 in a latch 634. This value is shown as C2. The value of C2 represents a delay, in the form of a number of selected transitions of the oscillator signal 506, between detection of the event 610 and the second selected transition 630 of SCL clock signal 606 following the first selected transition 622.

In some embodiments, information about an elapsed time between the Time Sync Marker and detection of the event 610 (i.e., time stamp of the event 610) may be based on the stored values C0, C1 and C2. In one or more embodiments, the time stamp 224 from FIG. 2 may be calculated at a slave device 204 by a time stamp calculation circuit 636 shown in FIG. 6 as:

$\begin{matrix} {{T\; 0} = {{C\; 0} + \frac{\left( {{C\; 2} - {{2 \cdot C}\; 1}} \right)}{\left( {{C\; 2} - {C\; 1}} \right)}}} & (1) \end{matrix}$ In equation (1), T0 represents the time stamp 224. The information about the time stamp of the event 610 may be communicated via the interface 228 of the slave device 204 to a master device 202 when SDA bus 206 is available. In an embodiment, as illustrated in FIG. 6, the time stamp value T0 defined by equation (1) may be also stored in a delay register 638 before being communicated to the master device 202. The delay register 638 may keep the time stamp value T0 until SDA bus 206 becomes available.

FIG. 7 is an example diagram 700 of capturing and reading time of events by a master device 702 from multiple slave devices 704 and 706, in accordance with embodiments of the present disclosure. The master device 702 may correspond to the master device 202 shown in FIG. 2, and each of the slave devices 704 and 706 may correspond to the slave device 204 shown in FIG. 2. As illustrated in FIG. 7, the master device 702 may broadcast a Time Sync CCC 708 to the slave devices 704, 706. The slave devices 704, 706 may track time delays 710, 712 between a Time Sync Marker (not shown in FIG. 7) generated when Sync CCC 708 is detected at the slave devices 704, 706 and detection of an event at each slave device. When an event 714 is detected at the slave device 704 and an event 716 is detected at the slave device 706, a time delay represented as a number of selected transitions of SCL clock signal (not shown in FIG. 7) tracked at each slave device is latched, i.e., the event is time-stamped in each slave device and stored in a delay register. As illustrated in FIG. 7, the slave device 704 may store the tracked delay 710 as the time stamp of the event 714 into the delay register 718; the slave device 706 may store the tracked delay 712 as the time stamp of the event 716 into the delay register 720. In one or more embodiments, the delay register 718 of the slave device 704 and the delay register 720 of the slave device 706 may correspond to the delay register 638 illustrated in FIG. 6.

In some embodiments, a slave device 204 shown in FIG. 2 may need to wait for a bus free condition on SDA bus 206 before a slave device can initiate an interrupt to a master device 202 shown in FIG. 2. As illustrated in FIG. 7, slave devices 704, 706 may need to wait until traffic 722 on SDA bus is finished. Then, the slave device 706 may initiate an in-band interrupt (IBI) 724 signaling to the master device 702 that the time stamp 712 of the event 716 is available to be read by the master device 702. Upon reception of the IBI 724, the master device 702 may send a request 726 via SDA bus to the slave device 706 requesting to read information about the time stamp 712 of the event 716 that is stored in the delay register 720 of the slave device 706. Upon reception of the request 726, the slave device 706 may read 728 the time stamp 712 from the delay register 720 and provide, via SDA bus, information about the time stamp 712 of the event 716 to the master device 702. After that, the master device 702 may initiate another read 730 from the delay register 720 of the slave device 704 that stores information about the time stamp 710 of the event 714. The information about the time stamp 710 of the event 714 may be then provided, via SDA bus, to the master device 702.

In the illustrative embodiment shown in FIG. 7, the slave device 706 may have a higher priority than the slave device 704. Although the slave device 704 may also initiate IBI, in this case there is no opportunity for the slave device 704 to do so because the master device 702 decides to read the time stamp 710 of the event 714 automatically in response to the IBI 724 received from the slave device 706. It should be also noted that because of the traffic 722 following Sync CCC 708, SCL clock signal (not shown in FIG. 7) may toggle continuously before and after the detected events 714, 716, thus providing a continuous time base for the slave devices 704, 706 to reference.

FIG. 8 is an example diagram 800 of capturing and reading time of events by a master device 802 and a monitor device 808 from multiple slave devices 804 and 806, in accordance with embodiments of the present disclosure. In some embodiments, the monitor device 808 may be interfaced via SDA bus and SCL bus with the slave devices 804 and 806. Unlike the master device 802, the monitor device 808 does not issue any commands nor generates/controls any clock signals. Instead, the monitor device 808 may simply monitor traffic on SDA bus and collect corresponding information communicated on the SDA bus by the master device 802 and/or the slave devices 804, 806. The master device 802 may correspond to the master device 202 from FIG. 2, and each slave device 804, 806 may correspond to the slave device 204 from FIG. 2.

As illustrated in FIG. 8, the master device 802 may broadcast Time Sync CCC 810 to the slave devices 804, 806 that track time delays 812 and 814 between a Time Sync Marker (not shown in FIG. 8) generated when Sync CCC 810 is detected at the slave devices 804, 806 and detection of an event at each slave device. Sync CCC 810 may be also detected by the monitor device 808. When an event 816 is detected at the slave device 804 and an event 818 is detected at the slave device 806, a time delay tracked at each slave device is latched, i.e., the event is time-stamped in each slave device and stored in a delay register. As illustrated in FIG. 8, the slave device 804 may store the tracked delay 812 between the Time Sync Marker and detection of the event 816 into a delay register 820; the slave device 806 may store the tracked delay 814 between the Time Sync Marker and the event 818 into the delay register 822. In one or more embodiments, the delay register 820 of the slave device 804 and the delay register 822 of the slave device 806 may correspond to the delay register 638 illustrated in FIG. 6.

As further illustrated in FIG. 8, other traffic 824 may be provided on SDA bus by the master device 802. The same traffic 824 may be also monitored by the monitor device 808. In some embodiments, each slave device may need to wait for a bus free condition on SDA bus before the slave device can initiate an interrupt to the master device. As illustrated in FIG. 8, the slave devices 804, 806 may need to wait until traffic 824 on SDA bus is finished. Then, the slave device 806 may initiate IBI signaling 826 via SDA bus that the time stamp 814 of the event 818 is available to be read. The same interrupt 826 sent via SDA bus may be received by both the master device 802 and the monitor device 808. Upon reception of the interrupt 826, the master device 802 may provide, to the slave device 806, a request 828 with an address of the slave device 806 requesting to read information about the time stamp 814 of the event 818 stored in the delay register 822 of the slave device 806. The request 828 comprising the address of the slave device 806 may be also received by the monitor device 808.

Upon reception of the request 828, the slave device 806 may read 830 the time stamp 814 from the delay register 822 and provide, via SDA bus, information about the time stamp 814 of the event 818 to the master device 802. At the same time, since the information about the time stamp 814 of the event 818 is available at SDA bus, the monitor device 808 may also obtain the time stamp 814 of the event 818. After that, the master device 802 may initiate, by sending a request 832 with an address of the slave device 804, another read 834 from the delay register 820 that stores information about the time stamp 812 of the event 816. The address 832 of the slave device 804 may be also received by the monitor device 808 that monitors all traffic on SDA bus. The information about the time stamp 812 of the event 816 may be then provided, via SDA bus, to the master device 802 and the monitor device 808. Upon reception of the time stamp data 812 and 814 from the slave devices 804 and 806, respectively, the master device 802 calculates time of the events 816 and 818 referenced to a global system reference clock signal, i.e., SCL clock signal (not shown in FIG. 8) generated and controlled by the master device 802, as discussed in more detail herein in relation to FIG. 2 and FIG. 12.

In some embodiments, the master device 802 is not capable of processing the time stamp data 812, 814, i.e., the master device 802 does not support converting the time stamp data 812, 814 into actual times of the events referenced to a global system reference clock signal. In this case, the monitor device 808 can be configured to handle processing of the time stamp data 812, 814 received from the slave devices 804, 806, thus allowing usage of a master device that does not support time-stamping. In this configuration, the master device 802 may still control SDA bus and SCL bus, as well as handle reads/writes/interrupts from/to the slave devices 804, 806, as discussed above. However, the master device 802 does not handle the intricacies of time-stamping. Instead, the monitor device 808 is configured to convert the received time stamp data 812, 814 into times of the events 816, 818 referenced to a global system reference clock signal. The monitor device 808 is configured to keep track of selected transitions of SCL clock signal and time the transitions of SCL clock signal to its own accurate time base, in the same way that the master device 802 would have done so, as discussed in more detail in relation to FIG. 2 and FIG. 12.

When the slave device 806 initiates IBI 826 by pulling down SDA bus during a bus-idle state after the traffic 824 is finished, the master device 802 responds by toggling SCL clock signal and initiates read-back of the time stamp information 814 from the slave device by sending the request 828. However, the master device 802 may ignore the received time stamp information 814. Instead, the master device 802 may rely on the monitor device 808 to also read the same time stamp data 814 and use the time stamp 814 to calculate an actual time of the event 818 detected at the slave device 806. Similarly, the monitor device 808 utilizes the time stamp 812 received from the slave device 804 and calculates a time of the event 816 detected at the slave device 804. At a later time, the monitor device 808 may send information about times of the events 816, 818 to the master device 802.

In the illustrative embodiment shown in FIG. 8, the slave device 806 may have a higher priority than the slave device 804. Although the slave device 804 may also initiate IBI, in this case there is no opportunity for the slave device 804 to do so because the master device 802 decides to read the time stamp 812 of the event 816 automatically in response to IBI 826 received from the slave device 806. It should be noted that because of the traffic 824 following Sync CCC 810, SCL clock signal (not shown in FIG. 8) may toggle continuously before and after the detected events 816, 818, thus providing a continuous time base for the slave devices 804, 806 to reference.

FIG. 9 is an example schematic of circuitry 900 for implementation of time-stamping at a slave device, such as the slave device 204 shown in FIG. 2 without the oscillator circuit 500 from FIG. 5 shown as a part of the time-stamping circuitry 600 in FIG. 6, in accordance with embodiments of the present disclosure. The circuitry 900 may be a part of the Time Tracking/Trigger Control circuit 214 of the slave device 204 shown in FIG. 2.

As discussed above with reference to the circuitry 400 illustrated in FIG. 4, a sync pulse 402 present at a reset line 902 may be generated when (Time) Sync CCC is detected, i.e., when a rising edge of pulse 406 shown in FIG. 4 is detected at an input 904. A falling edge of the sync pulse 402 may represent the Time Sync Marker that aligns with a selected transition 410 of SCL clock signal 906 during ‘T’ bit of a Time Stamp Sync command (e.g., SDR Time Sync command 210 broadcast from the master device 202 shown in FIG. 2, Time Stamp Sync command 300 shown in FIG. 3) following detection of Sync CCC at the input 904. The sync pulse present at the reset line 902 may reset a counter 908 to all zeroes. The counter 908 may correspond to the counter 414 of the circuitry 400 shown in FIG. 4. The counter 908 may be configured to increment on every selected transition of SCL clock signal 906, and may provide a uniform time reference across all slave devices while SCL clock signal 906 may be generated and controlled by a master device.

In some embodiments, an event 910 may be time stamped based at least in part on a value 912 of the counter 908. Upon detection of the event 910, the value 912 representing a number of selected transitions of SCL clock signal 906 between the Time Sync Marker and a last selected transition of SCL clock signal 906 prior to detection of the event 910 may be stored in a latch 914. As illustrated in FIG. 9, the value C0 representing the number of selected transitions of SCL clock signal 906 between the Time Sync Marker and detection of the event 910 is stored in the latch 914.

Since the oscillator circuit 500 comprising the burst oscillator 502 from FIG. 5 is not included in the circuitry 900 illustrated in FIG. 9, values of C1 and C2 associated with finer resolution of time-stamping are place-holders and set to zero. After reading time-stamp data 224 given by the value of C0 stored in the latch 914 associated with the time of the event 910, a master device 202 illustrated in FIG. 2 may translate, by the real time calculation unit 256, a value of C0+1 and a value of C0+2 into a system reference time for each value, i.e., into real times T1 and T2, respectively. In some embodiments, the value of C0+1 represents a number of selected transitions of SCL clock signal 906 between the Time Sync Marker and a first selected transition of SCL clock signal 906 following the event 910, and the value of C0+2 represents a number of selected transitions of SCL clock signal 906 between the Time Sync Marker and a second selected transition of SCL clock signal 906 following the event 910. The master device 202 may then determine, along with T1 and T2, a system reference (real) time T of the event 910. Hence,

$\begin{matrix} {T = {{{{T\; 1} - {\frac{{\left( {{T\; 2} - {T\; 1}} \right) \cdot C}\; 1}{\left( {{C\; 2} - {C\; 1}} \right)\mspace{11mu}}\mspace{14mu}{where}\mspace{14mu} C\; 2}} > {C\; 1\mspace{14mu}{else}\mspace{14mu} T}} = {T\; 1.}}} & (2) \end{matrix}$ In the illustrative embodiment shown in FIG. 9, both values of C1 and C2 are set to zeroes, and the real time T of the event 910 may be determined only based on the value of C0+1, i.e., the real time T of the event 910 may be equal to T1.

In accordance with embodiments of the present disclosure, as discussed above, multiple slave devices can initiate simultaneous operations (e.g., measurements) based on Time Sync triggering controlled by a master device via SDA bus. Based on this approach, additional communication channels between the master device and the slave devices can be eliminated. Embodiments of the present disclosure support usage of a time synchronization command broadcast by the master device that can start a timer at each slave device that triggers an event (e.g., measurement) at the end of a pre-determined time period. In one or more embodiments, a time delay for a triggering event at each slave device can be set by a command communicated by the master device via SDA bus that may precede the time synchronization command.

FIG. 10 is an example schematic of circuitry 1000 for implementation of delayed triggering at a slave device 204 illustrated in FIG. 2, in accordance with embodiments of the present disclosure. The circuitry 1000 may be a part of the Time Tracking/Trigger Control circuit 214 of the slave device 204 shown in FIG. 2. In some embodiments, a master device 202 illustrated in FIG. 2 may control an exact time of a trigger generated at each slave device 204.

A flip flop 1040 generates a sync pulse on a reset line 1002 when (Time) Sync CCC is detected at an input 1004, i.e., when the time synchronization command is detected. A falling edge of the sync pulse may represent a Time Sync Marker 1006 that aligns with a selected transition of SCL clock signal 1008 during ‘T’ bit of the time synchronization command (e.g., SDR Time Sync command 210 broadcast from the master device 202 shown in FIG. 2) following detection of the time synchronization command at the input 1004. The sync pulse present at the reset line 1002 may reset a counter 1010 to all zeroes. In an embodiment, the counter 1010 may correspond to the counter 414 of the circuitry 400 shown in FIG. 4. The counter 1010 increments on every selected transition of SCL clock signal 1010, and may provide a uniform time reference across all slave devices 204, wherein SCL clock signal 1008 may be generated and controlled by the master device 202.

The circuit 1000 illustrated in FIG. 10 may be generally configured to track a number of selected transitions of SCL clock signal 1008 after the time synchronization command is detected and to generate a trigger signal responsive to the number of selected transitions of SCL clock signal 1008 reaching a delay setting indicated by delay setting information 1012, which can be provided by the master device 202 into a delay register 1014. In some embodiments, the delay setting information 1012 may comprise coarse delay setting information 1016 and fine delay setting information 1018 that may be set in a command communicated by a the master device 202 via SDA bus prior to broadcasting Time Sync command. The coarse delay setting information 1016 indicates a trigger delay in the form of a number of selected transitions of SCL clock signal 1008 that are to occur between the Time Sync Marker 1006 and generation of the trigger signal. As illustrated in FIG. 10, a comparator 1020 may be configured to compare the coarse delay setting information 1016 and a value 1022 of the counter 1010 representing a number of selected transitions of SCL clock signal 1008 occurred after the Time Sync Marker 1006. When the value 1022 of the counter 1010 is equal to the coarse delay setting information 1016 and a tracked number of selected transitions of SCL clock signal 1008 reaches the coarse delay setting information 1016, the output of the comparator 1024 becomes a logical ‘1’. As a result, flip flop 1050 causes enable signal 1026 to become logical ‘1’ and enable operation of an oscillator and counter circuit 1028. The oscillator and counter circuit 1028 can be used in conjunction with the counter 1010 and the comparator 1024 to provide finer resolution for delayed triggering.

For some embodiments, the oscillator and counter circuit 1028 may correspond to the oscillator circuit 500 illustrated in FIG. 5, which comprises the burst oscillator 502 and the counter 504. When enabled by the enable signal 1026, the oscillator and counter circuit 1028 internally generates a burst oscillator signal 1030 with a frequency higher than a frequency of SCL clock signal 1008. As illustrated in FIG. 10, upon activation of the oscillator and counter circuit 1028 by the enable signal 1026, the burst oscillator within the oscillator and counter circuit 1028 may generate the burst oscillator signal 1030, whereas the counter within the oscillator and counter circuit 1028 may keep track of a number of selected transitions of the burst oscillator signal 1030. The fine delay setting information 1018 indicates a trigger delay in the form of a number of selected transitions of the burst oscillator signal 1030 that are to occur between the enable signal 1026 and generation of the trigger signal. Once the number of selected transitions of the burst oscillator signal 1030 represented by a signal 1032 at the output of the oscillator and counter circuit 1028 reaches the fine delay setting 1018, a comparator 1034 causes the logic level of the trigger signal 1036 to become a logical “1”. The trigger signal 1036 switches logic states at an exact time instant controlled by the master device 202 based on coarse and fine delay setting information. The trigger signal 1036 generated by the circuitry 1000 illustrated in FIG. 10 may correspond to the delayed trigger signal 238 generated by the Time Tracking/Trigger Control circuit 214 of the slave device 204 shown in FIG. 2. The delayed trigger signal 238 may initiate operation of the transducer 240 coupled to the slave device 204 at an exact time instant controlled by the master device 202.

FIG. 11 is an example diagram 1100 of controlling time of events at multiple slave devices by a master device, in accordance with embodiments of the present disclosure. As illustrated in FIG. 11, a master device 1102 may provide to slave devices 1104 and 1106 delay setting information, i.e., delay setting information 1108 may be stored at a delay register 1110 of the slave device 1104, and delay setting information 1112 may be stored at a delay register 1114 of the slave device 1106. The delay register 1110 of the slave device 1104 and the delay register 1114 of the slave device 1106 may correspond to the delay register 1014 shown in FIG. 10. In some embodiments, as discussed, delay setting information 1108 and 1112 may be communicated via SDA bus to the slave devices 1104 and 1106 via SDR commands sent from the master device 1102. The master device 1102 may correspond to the master device 202 from FIG. 2, and each slave device 1104, 1106 may correspond to the slave device 204 from FIG. 2.

As further illustrated in FIG. 11, following communication of delay setting information 1108 and 1112, the master device 1102 may broadcast via SDA bus a time synchronization command, Sync CCC 1116. Upon detection of Sync CCC 1116 at the slave devices 1104 and 1106, a Time Sync Marker (not shown in FIG. 11) may be generated at each slave device, i.e., the slave devices 1104 and 1106 may be synchronized by clearing their respective counters. Starting from the Time Sync Marker, the slave devices 1104 and 1106 may track reference time that may be provided by the master device 1102 via SCL clock signal. When the tracked time at the slave device 1104 reaches the delay setting information 1108, the slave device 1104 may generate a trigger in the form of a trigger event 1118, which may be delayed by a specific reference time 1120 from the Time Sync Marker. Similarly, when the tracked time at the slave device 1106 reaches the delay setting information 1112, the slave device 1106 may generate a trigger in the form of a trigger event 1122, which may be delayed by a specific reference time 1124 from the Time Sync Marker.

In some embodiments, as discussed, a master device 202 illustrated in FIG. 2 may track system reference time, starting from Time Sync Marker indicated by a time synchronization command, in order to convert a time stamp of an event detected at a slave device 204 into a system (real) time that is referenced to a global clock signal being generated and controlled by the master device 202. FIG. 12 is an example schematic of circuitry 1200 implemented at the master device 202 for supporting time stamping, in accordance with embodiments of the present disclosure. The circuitry 1200 illustrated in FIG. 12 may correspond to the time tracking circuit 234 of the master device 202 shown in FIG. 2.

Flip flop 1202 generates a sync pulse on reset line 1204 when a Sync CCC broadcast is indicated by a signal at input 1206 having a high logic level. A falling edge of the sync pulse may represent a Time Sync Marker that aligns with a selected transition of SCL clock signal 1208 during ‘T’ bit of a time synchronization command detected at the input 1206. The sync pulse present at the reset line 1204 may reset a counter 1210 to all zeroes. The counter 1210 may correspond to the counter circuit 248 of the master device 202 shown in FIG. 2. The counter 1210 increments on every selected transition of SCL clock signal 1208, and may provide a uniform time reference between the master device and all slave devices. In an embodiment, when the master device controls SCL bus (e.g., as in SDR and Dual Data Rate (DDR) modes), SCL clock signal 1208 may be derived from a reference clock by the clock generator 260 from FIG. 2 and controlled by the master device 202.

In some embodiments, frequency changes of SCL clock signal 1208 may be stamped using a value 1212 of the counter 1210. Once a change of frequency (COF) signal 1214 that indicates a change of frequency of SCL clock signal 1208 becomes logical ‘1’, the value 1212 of the counter 1210 may be stored into a latch 1216, indicated as value C0 in FIG. 12. In an embodiment, the value of C0 represents a number of selected transitions of SCL clock signal 1208 between the Time Sync Marker and a last selected transition of SCL clock signal 1208 prior to a first change of frequency of SCL clock signal 1208. Referring back to FIG. 2, the value C0 stored into the latch 1216 in FIG. 12 may correspond to the SCL count C0 stored in the latch 250 of the master device 202 upon COF signal 252 goes high.

In some embodiments, a register or look-up table 1218 may store information related to different periods associated with different frequencies of SCL clock signal 1208. For example, as illustrated in FIG. 12, bits T(1,0) may encode duration of a period when a frequency of SCL clock signal 1208 is 12 MHz; bits T(0,1) may encode duration of a period when a frequency of SCL clock signal 1208 is 1 MHz; and bits T(0,0) may encode duration of a period when a frequency of SCL clock signal 1208 is 400 KHz. A value 1220 encoded by bits T(0:m) at the output of the register 1218 may be used in conjunction with the value of C0 stored in the latch 1216 to provide the relationship between a number of selected transitions of SCL clock signal 1208 and real time reference. Once COF signal 1214 that indicates a change of frequency of SCL clock signal 1208 becomes logical ‘1’, the value 1220 encoded by bits T(0:m) at the output of the register 1218 may be stored into a latch 1222, indicated as value C1. Therefore, the value of C1 may represent a period of SCL clock signal 1208 prior to a change of frequency of SCL clock signal 1208. The latched values of C0 and C1 may provide information about system time reference between the Time Sync Marker and COF.

In some embodiments, as illustrated in FIG. 12, when the value 1212 indicating a number of selected transitions of SCL clock signal 1208 between the Time Sync Marker and a change of frequency of SCL clock signal 1208 and the value 1220 representing encoded period of a frequency of SCL clock signal 1208 prior to the change of frequency are stored as values C0 and C1 respectively, a next selected transition of SCL clock signal 1208 may cause a flip flop 1224 to produce an interrupt (INT) signal 1226 initiating storage of the values C0 and C1 into a cache or register file. After that, Clear signal 1228 may be pulsed, which may reset the latches 1216 and 1222, i.e., the latched values C0 and C1 are cleared after being stored into the cache or register file based on the INT signal 1226. Referring back to FIG. 2, the values of C0 and C1 stored into the cache or register file may correspond to CNT_1 and T1 values stored in the register file 254 of the master device 202 shown in FIG. 2 upon COF signal 252 goes high.

Referring back to FIG. 12, after the latches 1216, 1222 are reset, the circuitry 1200 may continue to track a number of selected transitions of SCL clock signal 1208 following the first change of frequency of SCL clock signal 1208 until a next change of frequency of SCL clock signal 1208. The latched values C0 and C1 in FIG. 12 may provide information about reference time between the Time Sync Marker and that next change of frequency of SCL clock signal 1208 indicated by COF signal 1214. In this way, the master device 202 in FIG. 2 can track reference time based on SCL clock signal 1208 generated and controlled by the master device 202 starting at the Time Sync Marker, and utilize this reference time information to correlate it with a time stamp of an event detected at a slave device 204 for calculation of real time referenced to SCL clock signal 1208 of occurrence of the event detected at the slave device 204.

Starting from the time of synchronization represented by the Time Sync Marker, the master device 202 stores a time of the Time Sync Marker and counts by the counter 1210 each selected transition of SCL clock signal 1208. The master device 202 stores, in the latches 1222 and 1216, a measure 1220 representing a frequency of SCL clock signal 1208 (i.e., value C1) and a count 1212 of selected transitions of SCL clock signal 1208 at which a frequency change of SCL clock signal 1208 occurs indicated by COF signal 1214 (i.e., value C0). Upon INT 1226 initiated by COF signal 1214, the stored values of C0 and C1 may be transferred from the latches 1216, 1222 into the cache or register file. Referring back to FIG. 2, the values of C0 and C1 transferred into the cache or register file may correspond to CNT_i and Ti values, respectively, which are stored in the register file 254 of the master device 202 each time when COF signal 252 goes high, providing reference time information. In some embodiments, as discussed, the master device 202 may receive, via SDA bus 206, the time stamp 224 of the event 222 detected at the slave device 204. The master device 202 may use reference time information stored in the register file 254, i.e., CNT_i and Ti values, to reconstruct a time instant of any selected transition of SCL clock signal without actually storing the time of each such transition. When the master device 202 receives the SCL clock signal count or the time stamp 224 of the event 222 detected at the slave device 204, the master device 202 correlates, at the real time calculation unit 256, the time stamp 224 and the reference time information of the register file 254 to determine the exact time instant 258 of the event 222 with respect to the system reference clock.

FIG. 13 is a diagram 1300 illustrating a method performed at a master device 202 illustrated in FIG. 2 for time stamping changes in SCL clock signal, which may be performed by the circuitry 1200 in FIG. 12, in accordance with embodiments of the present disclosure. As illustrated in FIG. 13, SCL clock signal 1302 driving SCL bus may change its frequency, which may be controlled by the master device 202. Furthermore, for some period of time, SCL clock signal 1302 may have no transitions, which corresponds to bus free condition. Thus, SCL clock signal 1302 is not periodic for a certain period of time during bus free condition, whereas SCL clock signal 1302 may become periodic again, as illustrated in FIG. 13. As discussed above in relation to the circuitry 1200 illustrated in FIG. 12, the master device 202 may time stamp a last selected transition of SCL clock signal 1302 prior to a change of frequency of SCL clock signal 1302. As illustrated in FIG. 13, a selected last transition 1304 related to a prior clock frequency 1306 of SCL clock signal 1302 may be time-stamped relative to a Time Sync Marker (not shown). After that, a last selected transition of SCL clock signal 1302 for a next clock frequency 1308 is also time-stamped, i.e., a last high-to-low transition 1310 of SCL clock signal 1302 before bus free condition 1312 is time-stamped. The time stamp 1310 together with the time stamp 1304 indicates time between two consecutive changes of frequency of SCL clock signal 1302. It should be noted that bus free condition 1312 when SCL clock signal 1302 is not periodic can be also considered as a change of frequency of SCL clock signal 1302 as the frequency of SCL clock signal 1302 actually changes from non-zero frequency 1308 to zero. Thus, a high-to-low transition 1314 of SCL clock signal 1302 when transitioning from bus free condition 1312 to a new clock frequency 1316 is also time-stamped. The time stamp 1314 together with the time stamp 1310 indicates duration of the bus free condition 1312.

In some embodiments, as discussed, a master device 202 illustrated in FIG. 2 can use the time-stamped selected transitions of SCL clock signal 1302 (e.g., the time-stamped transitions 1304, 1310, 1314, and so on) to determine a system reference time of any time-stamped event a slave device 204 had detected. The time stamps 1304, 1310, 1314 may correspond to the values CNT_i stored in the register file 254 of the master device 202. The master device 204 may correlate the time-stamp 224 of the event 222 detected at the slave device 204 with the time-stamped transitions 1304, 1310, 1314 of SCL clock signal including information about periods Ti of SCL clock signal, and determine by the real time calculation circuit 256 a system reference time 258 of the event.

Embodiments of the present disclosure relate to a method for translation of a system time base at a master device to a local time base at a slave device for time stamping and delayed triggering. Referring back to FIG. 2, a master device 202 may be configured to generate a reference SCL clock signal 208 that is also available at one or more slave devices 204. In one or more embodiments, the reference SCL clock signal 208 may have a certain resolution and may be translatable into a system time. The master device 202 may then provide an indication of synchronization in the form of Time Sync commands 210 and 300 shown in FIG. 2 and FIG. 3 on SDA bus 206. By providing the indication of synchronization, the master device 202 may also set a reference point on SDA bus 206, which can correspond to a selected transition of reference SCL clock signal 208 during Time Sync command. The reference point provided by the master device 202 may be received at each slave device 204 as Time Sync Marker aligned with a selected transition of reference SCL clock signal 208. In some embodiments, as discussed, in response to receiving the reference point, each slave device 204 may track an amount of time that has passed in a local time reference. In response to detecting an event at that slave device 204, an indication of the amount of local time that has passed when the event was detected can be loaded into a register and/or can be send to SDA bus 206. In addition, based on the reference point provided by the master device 202 and the reference SCL clock signal 208, each slave device 204 may generate a trigger signal at a time instant directly controlled by the master device 202 and referenced based on the system time base.

Embodiments of the present disclosure further relate to a method for translation of a local time base at a slave device to a system time base at a master device for time stamping. One or more slave devices 204 may monitor for occurrence of an event. At each slave device 204, as discussed, the occurrence of the event can be marked in a local time base, and time stamp of the event can be latched at the slave device 204. Before starting monitoring for occurrence of an event, each slave device 204 may receive from the master device 202 via SDA bus 206 a reference point signal in a form of Time Sync Marker. The Time Sync Marker may be based on a reference clock, such as SCL clock signal 208, generated and controlled at the master device 202, and may be therefore translatable into a system-wide time base. At each slave device 204, a latency can be determined between a time when the Time Sync Marker is received at that slave device 204 and a time when the occurrence of the event is detected in the local time base. The latency corresponds to the time of the event in the local time base and can be reported to the master device 202. In some embodiments, as discussed, the master device 202 may determine respective times in the system-wide time base of occurrence of each of the events at slave devices 204.

FIG. 14 is a flow chart illustrating a method 1400 for time stamping that may be performed at a master device 202 illustrated in FIG. 2, in accordance with embodiments of the present disclosure.

Operations of the method 1400 may begin by the master device 202 generating 1402 a clock signal (e.g., SCL clock signal 208) and a synchronization command, such as Time Sync command 210.

The master device 202 transmits 1404 the clock signal and the synchronization command via the communication link, such as the communication link 205 illustrated in FIG. 2 that comprises SCL line 208 and SDA line 206.

The master device 202 receives 1406 timestamp information (e.g., time stamp 224) via the communication link, the timestamp information indicative of a number of selected transitions of the clock signal that elapse between the synchronization command and a time instant when an event is detected at the slave device (e.g., the event 222 detected at the slave device 204).

The Time Tracking circuit 234 of the master device 202, shown in more detail as the circuitry 1200 in FIG. 12, tracks 1408 counts of selected transitions of the clock signal between the synchronization command and frequency changes of the clock signal occurring after the synchronization command.

The real time calculation unit 256 of the master device 202 determines 1410 a time of the event detected at the slave device based on the timestamp information and the counts of the selected transitions of the clock signal.

FIG. 15 is a flow chart illustrating a method 1500 for delayed triggering that may be performed at a slave device 204 illustrated in FIG. 2, in accordance with embodiments of the present disclosure.

Operations of the method 1500 may begin by the slave device 204 receiving 1502, via a communication link that carries a clock signal (e.g., SCL clock signal 208), a synchronization command (e.g., Time Sync command 210) and delay setting information that may be provided by the SDR command 210 generated by the master device 202 prior to the Time Sync command. In some embodiments, as discussed, the communication link may correspond to the communication link 205 that comprises SDA line 206 and SCL line 208.

The Time Tracking/Trigger Control circuit 214 of the slave device 202, shown in more detail as the circuitry 1000 in FIG. 10, tracks 1504 a number of selected transitions of the clock signal (e.g., falling edges of SCL clock signal 208) after the synchronization command.

The slave device 204 generates 1506 a trigger signal, such as the delayed trigger 238, responsive to the number of selected transitions reaching a delay setting indicated by the delay setting information.

FIG. 16 is a flow chart illustrating a method 1600 for delayed triggering that may be performed at a master device 202 illustrated in FIG. 2, in accordance with embodiments of the present disclosure.

Operations of the method 1600 may begin by the master device 202 transmitting 1602, via a communication link, a clock signal (e.g., SCL clock signal 208) and a synchronization command (e.g., Time Sync command 210). In some embodiments, as discussed, the communication link may correspond to the communication link 205 that comprises SDA line 206 and SCL line 208.

The master device 202 transmits 1604 delay setting information indicating a number of selected transitions of the clock signal that are to occur between the synchronization command and generation of a trigger signal (e.g., the delayed trigger 238) at one or more slave devices 204 coupled to the communication link. In some embodiments, as discussed, the delay setting information may comprise coarse and fine delay setting information located in the SDR command 210 generated by the master device 202 prior to the synchronization command.

The foregoing description of the embodiments of the disclosure has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

Some portions of this description describe the embodiments of the disclosure in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software, firmware, hardware, or any combinations thereof.

Any of the steps, operations, or processes described herein may be performed or implemented with one or more hardware or software modules, alone or in combination with other devices. In one embodiment, a software module is implemented with a computer program product comprising a computer-readable medium containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability.

Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the disclosure, which is set forth in the following claims. 

What is claimed is:
 1. A system comprising: a master device coupled to a communication link, the master device to transmit, via the communication link, a clock signal and a synchronization command; and one or more slave devices coupled to the communication link, each slave device to: track a number of selected transitions of the clock signal between detection of the synchronization command at the slave device and detection of an event at the slave device, and generate information about an elapsed time between the detection of the synchronization command and the detection of the event by at least counting the number of selected transitions of the clock signal, the master device to: obtain the information about the elapsed time, and derive a time the event was detected at the slave device based on the information.
 2. The system of claim 1, wherein the master device is to receive the information about the elapsed time, and to determine the time the event was detected at the slave device based on the received information about the elapsed time and numbers of selected transitions of the clock signal occurring between frequency changes of the clock signal that are tracked at the master device.
 3. The system of claim 1, wherein the communication link comprises a single clock line that carries the clock signal and a single data line that carries the synchronization command.
 4. The system of claim 1, wherein: the one or more slave devices comprise a plurality of slave devices, each slave device of the plurality of slave devices is coupled to a corresponding sensor, and the event detected at a slave device corresponds to detection of physical phenomena by the corresponding sensor.
 5. The system of claim 1, wherein: each slave device comprises an oscillator circuit to generate an oscillator signal having a frequency higher than a frequency of the clock signal, and each slave device is to count a number of selected transitions of the oscillator signal occurring after the event is detected at the slave device and to generate the information about the elapsed time based on the number of selected transitions of the oscillator signal.
 6. The system of claim 1, wherein: the slave device is to generate an interrupt responsive to detection of the event at the slave device; the master device is to generate, responsive to the interrupt, a request for reading the information about the elapsed time from a register of the slave device; the slave device is to generate, in response to the request, the information about the elapsed time for the master device; and the master device is to store, in a register of the master device, the information about the elapsed time obtained from the slave device via the communication link.
 7. The system of claim 1, wherein: the slave device is to generate an interrupt responsive to detection of the event at the slave device; the master device is to generate, responsive to the interrupt, a request for reading the information about the elapsed time from a register of the slave device and to generate another request for reading other timestamp information from a register of another slave device of the one or more slave devices; the slave device is to generate, in response to the request, the information about the elapsed time for the master device; the another slave device is to generate, in response to the another request, the other timestamp information for the master device; the master device is to store, in a register of the master device, the information about the elapsed time obtained via the communication link from the slave device; and the master device is to store, in another register of the master device, the other timestamp information obtained via the communication link from the another slave device.
 8. The system of claim 7, further comprising: a monitor device coupled to the communication link, the monitor device to: receive the information about the elapsed time and store the information about the elapsed time into a register of the monitor device; receive the other timestamp information and store the other timestamp information into another register of the monitor device; and send the information about the elapsed time and the other timestamp information to the master device.
 9. An apparatus comprising: an interface circuit for coupling to a communication link, the interface circuit to: transmit, via the communication link, a clock signal and a synchronization command; and receive, via the communication link, timestamp information indicative of a number of selected transitions of the clock signal that elapse between the synchronization command and a time instant when an event is detected at a slave device; a time tracking circuit to track counts of selected transitions of the clock signal between the synchronization command and frequency changes of the clock signal occurring after the synchronization command; and a time calculation circuit to determine a time of the event detected at the slave device based on the timestamp information and the counts of the selected transitions of the clock signal.
 10. The apparatus of claim 9, wherein: the time tracking circuit is to count a first number of selected transitions of the clock signal between the synchronization command and a first frequency change of the clock signal and to count a second number of selected transitions of the clock signal between the synchronization command and a second frequency change of the clock signal, and the time calculation circuit is to determine the time of the event based on the first count of selected transitions of the clock signal and the second count of selected transitions of the clock signal.
 11. The apparatus of claim 10, wherein: the time tracking circuit is to store information about a period of the clock signal prior to the first frequency change of the clock signal and to store information about a second period of the clock signal prior to the second frequency change of the clock signal, and the time calculation circuit is to determine the time of the event further based on the first period of the clock signal and the second period of the clock signal.
 12. The apparatus of claim 9, wherein the communication link comprises a single wire clock line that carries the clock signal and a single wire data line that carries the synchronization command, and the interface circuit is configured to couple to the single wire clock line and the single wire data line.
 13. The apparatus of claim 9, wherein the interface circuit is to: receive, via the communication link, an interrupt from the slave device; transmit, via the communication link in response to the received interrupt, a request for reading the timestamp information from a register of the slave device; and store, in a register of the apparatus, the timestamp information obtained via the communication link.
 14. The apparatus of claim 9, wherein the interface circuit is to: receive, via the communication link, an interrupt from the slave device; transmit, via the communication link in response to the received interrupt, a request for reading the timestamp information from a register of the slave device and another request for reading other timestamp information from a register of another slave device; receive, via the communication link, the timestamp information from the slave device responsive to the request and the other timestamp information from the another slave device responsive to the another request; store, in a register of the apparatus, the timestamp information obtained via the communication link; and store, in another register of the apparatus, the other timestamp information obtained via the communication link.
 15. An apparatus comprising: an interface circuit for coupling to a communication link that carries a clock signal; and a control circuit to: track a number of selected transitions of the clock signal on the communication link between detection of a synchronization command received via the communication link and detection of an event, and generate information about an elapsed time between the detection of the synchronization command and the detection of the event by at least counting the number of selected transitions of the clock signal, the interface circuit to transmit, via the communication link to another apparatus, the information about the elapsed time.
 16. The apparatus of claim 15, wherein the control circuit further comprises: a sync pulse circuit to generate, based on the synchronization command, a sync pulse having a selected transition aligned with a selected transition of the clock signal; a counter circuit to increment, after initialization by the sync pulse, a value of the counter circuit on every selected transition of the clock signal; and a latch circuit to store, in response to the event, the value of the counter circuit, wherein the information about the elapsed time is based at least in part on the value of the counter circuit.
 17. The apparatus of claim 15, wherein the control circuit further comprises: an oscillator circuit to generate an oscillator signal having a frequency higher than a frequency of the clock signal; and the control circuit tracks a first number of selected transitions of the oscillator signal occurring after the event, and wherein the information about the elapsed time is further based on the first number of selected transitions.
 18. The apparatus of claim 15, wherein the control circuit further comprises: an oscillator circuit to generate an oscillator signal having a frequency higher than a frequency of the clock signal; and the control circuit is to track a first number of selected transitions of the oscillator signal between the detection of the event and a first selected transition of the clock signal that follows the event, the control circuit is to track a second number of selected transitions of the oscillator signal between the detection of the event and a second selected transition of the clock signal that follows the first selected transition of the clock signal, and wherein the information about the elapsed time is further based on the first number of selected transitions and the second number of selected transitions.
 19. The apparatus of claim 15, further comprising an event detector to detect the event from a sensor input associated with the apparatus.
 20. The apparatus of claim 15, wherein the communication link comprises a single wire clock line that carries the clock signal and a single wire data line that carries the synchronization command, and the interface circuit is configured to couple to the single wire clock line and the single wire data line.
 21. The apparatus of claim 15, wherein the interface circuit is to: send, via the communication link, an interrupt upon detection of the event; receive, via the communication link, a request transmitted in response to the interrupt to read the information about the elapsed time; and send, via the communication link in response to the request, the information about the elapsed time.
 22. A method comprising: generating a clock signal and a synchronization command; transmitting, via a communication link, the clock signal and the synchronization command; receiving, via the communication link, timestamp information indicative of a number of selected transitions of the clock signal that elapse between the synchronization command and a time instant when an event is detected at a slave device; tracking counts of selected transitions of the clock signal between the synchronization command and frequency changes of the clock signal occurring after the synchronization command; and determining a time of the event detected at the slave device based on the timestamp information and the counts of the selected transitions of the clock signal.
 23. The method of claim 22, further comprising: counting a first number of selected transitions of the clock signal between the synchronization command and a first frequency change of the clock signal; counting a second number of selected transitions of the clock signal between the synchronization command and a second frequency change of the clock signal; and determining the time of the event based on the first count of selected transitions of the clock signal and the second count of selected transitions of the clock signal.
 24. The method of claim 22, further comprising: storing information about a period of the clock signal prior to the first frequency change of the clock signal; storing information about a second period of the clock signal prior to the second frequency change of the clock signal; and determining the time of the event further based on the first period of the clock signal and the second period of the clock signal.
 25. The method of claim 22, further comprising: receiving, at a master device via the communication link, an interrupt from the slave device; transmitting, via the communication link in response to the received interrupt, a request for reading the timestamp information from a register of the slave device; and storing, in a register of the master device, the timestamp information obtained via the communication link.
 26. The method of claim 22, further comprising: receiving, via the communication link, an interrupt from the slave device; transmitting, via the communication link in response to the received interrupt, a request for reading the timestamp information from a register of the slave device and another request for reading other timestamp information from a register of another slave device; receiving, via the communication link, the timestamp information from the slave device responsive to the request and the other timestamp information from the another slave device responsive to the another request; storing, in a register of a master device, the timestamp information obtained via the communication link; and storing, in another register of the master device, the other timestamp information obtained via the communication link. 